1. Field of the Invention
The present invention relates to a fixed chip resistor or a thick-film type electrical resistor provided in circuit boards and the like and, more particularly, to an electrical resistor capable of being obtained by sintering in a non-oxidizing atmosphere. The present invention also relates to a method for making such resistors.
2. Statement of the Prior Art
Electrical circuits of electronic equipment are generally constructed by mounting various electrical elements such as resistors, capacitors, diodes and transistors to circuit boards. With miniaturization of electronic equipment, however, much use has been made of circuit boards capable of increasing the density of such mounted electrical elements.
The resistors mounted to such circuit boards include a thick-film resistor formed by printing and firing a paste of a resistor material directly onto a circuit, a fixed chip resistor made by forming such a thick-film resistor across a pair of electrode terminals of a rectangular ceramic chip, and the like.
Hitherto, such a thick-film resistor has generally been formed on a circuit board in the following manner. A paste of a conductor material such as Ag or Ag-Pd is applied and fired on the surface of an alumina substrate obtained by sintering at, e.g., about 1500.degree. C. Afterwards, a paste containing, e.g., RuO.sub.2 as the main material of the resistor is applied on that surface by means of screen printing, etc., followed by firing at 750.degree. to 850.degree. C. and, if required, adjustment of a resistance value by means of laser trimming, etc.
However, recent heavy demands for reduction in the weight, thickness, size and cost of electronic equipment, etc. have led to intensive studies for reducing in the size and cost of circuit boards.
Referring to concrete measures to meet the former size reduction, multilayered circuit boards deserve the first mention, and formed resistors the second mention. Known examples of multilayered circuit boards include a multilayered circuitry board obtained by laminating ceramic green sheets, each having a paste of a conductor material such as Ag or Ag-Pd printed thereon, and simultaneously sintering them at 800.degree. to 1100.degree. C. in the air, while known examples of the formed resistor include a multilayered with a formed resistor, obtained by printing a paste of a RuO.sub.2 base resistor material on a ceramic green sheet having said paste of a conductive material printed thereon, laminating such sheets, and then simultaneously sintering them.
Referring to concrete measures to achieve the latter cost reduction, multilayered circuitry boards have been put to practical use, and are obtained by using conductive materials based on inexpensive base metals such as Ni or Cu in place of those based on noble metals such as Ag or Ag-Pd, and sintering them simultaneously with green ceramic at 800.degree. to 1100.degree. C. in a neutral or reducing atmosphere to avoid any increase in resistance due to their oxidation, such as a nitrogen gas or a hydrogen-containing nitrogen gas. As disclosed in Japanese Patent Laid-Open (Kokai) Publication No. 56-153702 in particular, there are also known thick-film resistors, etc. which are obtained by applying a resistor material comprising MoSi.sub.2 --TaSi.sub.2 and glass on an alumina substrate including a copper (Cu) conductor followed by a heat treatment.
Where it is intended to simultaneously reduce the size and cost of circuit boards, the RuO.sub.2 base resistor material undergoes a reducing reaction, when it is sintered simultaneously with green ceramic in a nitrogen gas or hydrogen-containing nitrogen atmosphere, and it does not provide any resistor.
Simultaneously sintering of the resistor material comprising MoSi.sub.2 --TaSi.sub.2 and glass and the green ceramic sheet in a non-oxidizing atmosphere also offers the problems that the substrate may warp due to a difference in the dislocation shrinkage curve, or may tend to swell easily due to the gas generated by the decomposing reaction of MoSi.sub.2 --TaSi.sub.2. To solve such problems, it is known by way of example to use a resistor material comprising MoSi.sub.2 -salts of metal fluorides (e.g. calcium fluoride) and glass, as disclosed in Japanese Patent Laid-Open (Kokai) Publication No. 60-198703. In this example, such warping or swelling of the substrate as mentioned above is not found.
However, when allowed to stand alone in a relative humidity of 95% for 1000 hours, the thick-film resistor obtained by applying such a resistor material comprising MoSi.sub.2 -metal fluorides and glass on a green ceramic sheet and simultaneously sintering them shows a 5 to 10% increase in the resistance value and, hence, cannot perform its own resistor function.
Further, the conventional electrical resistors as mentioned above have posed some problem, when used as the resistor element for a circuit needing precise work, since it is impossible to decrease the temperature dependence coefficient of their resistance value to 1000 ppm/.degree.C. or lower.